Electrophotographic image forming apparatuses are provided with a charging device that uniformly charges the photosensitive surface of an image carrier (for example, a photosensitive drum), a latent image forming device that forms an electrostatic latent image on the charged photosensitive surface according to image information, and a developing device that develops the electrostatic latent image. Furthermore, the image forming apparatuses are also provided with a transfer device that transfers the electrostatic latent image developed with a developer onto recording paper, and successively performs an image forming process while rotating the photosensitive surface of the photosensitive drum. With such image forming apparatuses, variations occur in image density and gray scale reproduction properties due to the influence of short-term variations resulting from a variation in the installation environment of the apparatus and a variation in the internal environment of the apparatus, and long-term variations resulting from changes over time (degradation over time) of the photosensitive drum and the developer. In other words, in order to output images of uniform density and gray scale reproduction properties, corrections need to be made as appropriate taking such variations into consideration.
To address the problems described above, Japanese Patent Laid-Open No. 2007-298949 proposes an image forming apparatus that controls the amount of light and the duration of light emission in consideration of the spot size of a laser. With this configuration, it is possible to obtain a relationship between development contrast and a plurality of density patches truly representing the development characteristics of the image forming apparatus in a short time without changing a charging bias and a developing bias. Accordingly, with Japanese Patent Laid-Open No. 2007-298949, appropriate values for the charging bias and the developing bias can be obtained from the obtained relationship, so that control of dense areas can be performed with high accuracy.
However, the technique described above has the following problems. For example, with the technique disclosed in Japanese Patent Laid-Open No. 2007-298949, if a toner charge amount is lower than a desired level when obtaining appropriate setting values for the charging bias and the developing bias from the relationship between density patch and development contrast, images are formed with a density higher than a predetermined level. For this reason, a control device controls contrast so as to suppress the amount of toner developed. When the toner charge amount is not at a desired level, if the user starts printing after density stabilization control has been performed in which appropriate setting values for the charging bias and the developing bias have been obtained, the toner charge amount changes due to friction between the toner and the carrier. When the toner consumption amount is low, the toner charge amount increases, and the density decreases accordingly. Consequently, appropriate setting values for the charging bias and the developing bias cannot be obtained with a desired charge amount. As described above, with the method in which the contrast potential is set based on the toner density, it may not be possible to output images at a desired density due to a change in the toner charge amount.